The array is multiplied with the fourier transform of a ellipsoid of
given sizes. If the parameter n is negative, then the input is assumed
to be the result of a complex fft. If n is larger or equal to zero, the
input is assumed to be the result of a real fft, and n gives the length
of the of the array before transformation along the the real transform
direction. The axis of the real transform is given by the axis
parameter. This function is implemented for arrays of
rank 1, 2, or 3.